As cannabis legalization has increased rates of adult cannabis use, it likely has increased rates of driving under the influence of cannabis. Presently, it is difficult to say by how much.
As more US states legalize medicinal and recreational cannabis, reported rates of driving under the influence of the drug are increasing significantly. However, this observation is complicated by the fact that blood THC levels can be detectable for weeks following cannabis use, even when users are not currently intoxicated or impaired. In addition, there are no well-established set of procedures or tests at this time for detecting drivers at the roadside who are impaired due to cannabis use.
The best epidemiologically derived estimates are that cannabis-impaired motor vehicle drivers have double the odds of being involved in a motor vehicle crash. This should be contrasted with those of alcohol which are approximately 20-times higher than baseline at a blood/breath alcohol concentration of 0.10.
Because of the public health implications, states and law enforcement agencies are seeking guidance on how to develop valid and reliable field tests for detecting cannabis-impaired drivers, analogous to those used currently to identify alcohol intoxication. However, the significantly different pharmacokinetics and behavioral impairment profiles produced by THC versus alcohol make achieving this goal extremely difficult. As a shortcut, some states have adopted so-called “per-se” rules, whereby a specified blood THC level in and of itself equates to cannabis-related impairment. However, there is little scientific foundation for such legislation.
There are many, often unacknowledged, research gaps in these and related areas that need to be addressed in order provide a more coherent basis for public policies. Issues of driver impairment following THC-containing edibles, and impairment levels following alcohol/cannabis combinations for example are almost completely undocumented at present.
The situation for cannabis mirrors that of many prescription medications, such as benzodiazepines, opioids, and hypnotics (e.g. Ambien). There is presently no empirically valid measure of quantifying the level of acute intoxication and impairment from these medications. Society has accepted this limitation and has adapted by training police officers to evaluate an individual for the acute effects of intoxication to the best of their ability.
A recent review discusses many of the above issues in detail: